When designing microwave circuits, microstrip transmission lines are commonly used. A microstrip transmission line comprises a metal ground plane and a conductor, where a dielectric carrier material is positioned between the metal ground plane and the conductor. This configuration is economical and relatively easy to design.
Another type of transmission line is a stripline conductor. Here, a conductor is sandwiched between two dielectric carrier materials, where ground planes are placed on the sides of the dielectric carrier materials that face away from the conductor.
Yet another type of transmission line is a co-planar conductor, where a conductor is placed on a dielectric carrier material and ground planes are placed on the same side of the dielectric carrier material as the conductor, surrounding it, with a small gap between the ground plane and the conductor.
However, due to losses in the dielectric carrier material, it is sometimes not possible to use any of the transmission lines above. When there for example is a filter in the layout, the filter may have to be realized in waveguide technology. Waveguides are normally filled with air or other low-loss materials.
When there is a filter in a microwave circuit microstrip layout, the filter may thus be realized by means of a waveguide filter in order to lower the losses. In that case, there has to be corresponding microstrip to waveguide transitions at the ends of the filter. Such a waveguide is preferably surface-mounted, enabling it to be mounted to the dielectric carrier material.
Such a surface-mounted waveguide is normally made having three walls and one open side. Metalization is then provided on the side of the dielectric carrier material facing the waveguide, where the metalization serves as the remaining wall of the waveguide, thus closing the waveguide structure when the waveguide is fitted to the dielectric carrier material.
Another application for surface-mounted waveguides is when there has to be a microstrip to waveguide transition in the form of a bend, allowing a waveguide to be mounted to the dielectric carrier material in such a way that the waveguide extends essentially perpendicular to the main surfaces of the dielectric carrier material.
It is also conceivable that a waveguide filter is realized having a separate fourth closing wall made as a metalization on a dielectric carrier material, where such a design is found cost-effective.
It is of course also common that it is desired to have a transition from a transmission line to a general waveguide interface.
A special case regarding surface-mountable waveguides is disclosed in the paper “Surface-mountable metalized plastic waveguide filter suitable for high volume production” by Thomas J Müller, Wilfried Grabherr, and Bernd Adelseck, 33rd European Microwave Conference, Munich 2003. Here, a surface-mountable waveguide is arranged to be mounted on a so-called footprint on a circuit board. A microstrip conductor to waveguide transition is disclosed, where the end of the microstrip conductor acts as a probe for feeding the waveguide's opening. The microstrip conductor is in contact with the waveguide via a stepped ridge, which matches the impedance in the transition. Furthermore, the transition region is bordered by via holes.
There is, however, a problem with the design according to the paper, as well as with general transitions from a transmission line to a waveguide interface, since a microstrip probe is carried by the circuit board, causing losses, and since there is a need of via holes, defining an electric wall through the circuit board.
There is thus a demand for a waveguide arrangement comprising a transmission line to waveguide transition that provides lower losses and a less expensive and simpler design.